Molybdenum is an essential element in enzymes involved in purine metabolism, sulfur metabolism, and the assimilation of inorganic nitrogen. Exposure to excess molybdenum has been linked with gout and atherosclerosis. Molybdenum levels in the soil affect the levels of nitrate and nitrite and interfere with normal copper metabolism. Current evidence indicates that molybdoenzymes fall into two classes. This research is related to the class of molybdoenzymes which includes the xanthine oxidases, the sulfite oxidases and the nitrate reductases. These enzymes contain a common molybdenum cofactor which is thought to be a monomeric molybdenum center with ligating sulfur atoms. The objectives of this research are to investigate structural models for the molybdenum centers of these enzymes by an integrated program of synthesis and chemical, spectroscopic and x-ray structural studies on well-defined molybdenum compounds. Special emphasis will be given to monomeric molybdenum(V) compounds, compounds which contain sulfido or hydrosulfido groups attached to molybdenum, and compounds in which the coordination environment about the metal is controlled by sterically constraining ligands. Compounds will be characterized by electron paramagnetic spectroscopy (EPR), 95Mo nuclear magnetic resonance spectroscopy (NMR), and x-ray crystallography.